To produce a steel plate having a tensile strength of the 780 MPa class and having excellent low temperature toughness, refinement of the quenched structure (lower bainite or martensite) is said to be effective. To refine a quenched structure, it is necessary to refine the austenite grain size before the formation of the quenched structure before cooling the steel material.
In particular, when producing a plate by direct quenching (DQ), controlled rolling may be used to control the austenite grain size. By rolling in the austenite recrystallization region, refinement of the austenite grain size before the formation of the quenched structure becomes possible.
However, it is difficult to obtain a grasp of the austenite recrystallization region and pre-recrystallization region of austenite of a steel before rolling. Variation in the austenite grains is liable to invite instability in the quality of steel.
On the other hand, by making maximum use of controlled rolling and refining the structure, excellent low temperature toughness should be able to be secured. For example, Japanese Patent Publication (A) No. 6-240355 discloses performing final rolling of a steel plate containing Nb at the pre-recrystallization region of austenite of 780° C. or less so as to achieve refinement of structure of thick-gauge steel plate and secure excellent low temperature toughness at the center of plate thickness.
However, with this method of production, the quenchability greatly falls and a ferrite structure is mainly formed, so it is difficult to secure a 780 MPa class high strength and high toughness. Furthermore, rolling at a low temperature becomes necessary, so there is also a problem from the viewpoint of the productivity.
Further, the Nb added for refining the structure is extremely high in effect of hardening the welding heat affected zone (HAZ). As a result, it causes deterioration of the HAZ toughness. In particular, with high strength steel such as the 780 MPa class steel, the deterioration in HAZ toughness due to this effect becomes an extremely great problem.
To obtain a 780 MPa class strength, it is effective to add B having a large effect in raising the quenchability. However, as described in Japanese Patent Publication (A) No. 2007-138203, B promotes the formation of a hardened second phase due to the simultaneous addition of Nb. The deterioration of the HAZ toughness became a particular problem as a result.
To improve the HAZ toughness, it is known that addition of Ti is effective. This is because Ti bonds with N etc. to form fine precipitates and has the effect of restraining grain growth. However, as described in Japanese Patent Publication (A) No. 2000-8135, in the case of steel containing C in 0.2% or more for the purpose of securing the strength, extremely hard grains of TiC are formed at the base metal and HAZ. This has the problem of causing a deterioration of toughness.
In the above way, up to now, the fact is that no method of production of 780 MPa class high strength steel plate free of Nb, free of Ti, and provided with both high strength and excellent low temperature toughness has yet been proposed.